SPI Explained

Introduction:-

Serial Peripheral Interface (SPI) is a synchronous serial data protocol used by microcontrollers for communicating with one or more peripheral devices quickly over short distances. It can also be used for communication between two microcontrollers.

With an SPI connection there is always one master device (usually a microcontroller) which controls the peripheral devices. Typically there are three lines common to all the devices:

• MISO (Master In Slave Out) - The Slave line for sending data to the master,
• MOSI (Master Out Slave In) - The Master line for sending data to the peripherals,
• SCK (Serial Clock) - The clock pulses which synchronize data transmission generated by the master

and one line specific for every device:

• SS (Slave Select) - the pin on each device that the master can use to enable and disable specific devices.

When a device's Slave Select pin is low, it communicates with the master. When it's high, it ignores the master. This allows you to have multiple SPI devices sharing the same MISO, MOSI, and CLK lines.

To write code for a new SPI device you need to note a few things:

• What is the maximum SPI speed your device can use? This is controlled by the first parameter in SPISettings. If you are using a chip rated at 15 MHz, use 15000000. Arduino will automatically use the best speed that is equal to or less than the number you use with SPISettings.
• Is data shifted in Most Significant Bit (MSB) or Least Significant Bit (LSB) first? This is controlled by second SPISettings parameter, either MSBFIRST or LSBFIRST. Most SPI chips use MSB first data order.
• Is the data clock idle when high or low? Are samples on the rising or falling edge of clock pulses? These modes are controlled by the third parameter in SPISettings.

The SPI standard is loose and each device implements it a little differently. This means you have to pay special attention to the device's datasheet when writing your code.

Generally speaking, there are four modes of transmission. These modes control whether data is shifted in and out on the rising or falling edge of the data clock signal (called the clock phase), and whether the clock is idle when high or low (called the clock polarity). The four modes combine polarity and phase according to this table:

Mode	Clock Polarity	Clock Phase	Output Edge	Data Capture
SPI_MODE0	0	0	Falling	Rising
SPI_MODE1	0	1	Rising	Falling
SPI_MODE2	1	0	Rising	Falling
SPI_MODE3	1	1	Falling	Rising

So Let's see an Example on how SPI and Arduino(AVR Microcontollers) work Hand in Hand

Working:-

Now, what we're gonna do is take an Arduino and call it as Master and take another Arduino and call it as Slave .The Master unit sends a message (Eg: Hello World) to the Slave unit, which waits for the data. As soon as data is arrived, the "process" variable becomes true, indicating there is data in buffer. In the Main loop we read the Buffer and print it in the Serail Terminal.

Demo:-

Circuit:-

Code:-

1. Master Unit Code : https://gist.github.com/TheMicroKidBlog/a08c3824705c11780347e4f82969ba7f#file-nano_spi_mastercode

2. Slave Unit Code : https://gist.github.com/TheMicroKidBlog/a08c3824705c11780347e4f82969ba7f#file-nano_spi_slavecode

Result:-

Here is the Final Output of the Slave Unit in the Serial Monitor